Polymer Gels - American Chemical Society

Chapter 2

Transient Network of Random Copolymers of Sodium 2-(Acrylamido)-2-methylpropanesulfonate and Associative Macromonomers Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch002

Yotaro Morishima, Tetsuya Noda, and Akihito Hashidzume D e p a r t m e n t o f M a c r o m o l e c u l a r S c i e n c e , G r a d u a t e S c h o o l of S c i e n c e , Osaka University, Toyonaka, O s a k a 560-0043, Japan

Micelle formation and rheological behavior in water of a sodium polysulfonate with C E surfactant moieties, HO(CH CH O) C H where m = 6 or 25, pendent to the polymer are discussed. The polymer-bound C E moieties (contents ranging 10 - 30 mol %) undergo side -chain micellization through intra- and interpolymer associations, forming polymer networks. Aggregation numbers of the polymer-bound C F moieties in one micelle unit are similar to those of free C F surfactants. The degree of crosslinking by the side-chain micellization was significantly greater for larger m, increasing with polymer concentration (C ). Arising from the dynamic nature of micelles as junctions, polymer solutions exhibit viscoelastic behavior, concentrated solutions showing gel -like behavior characteristic of transient networks. 12

2

2

m

12

m

25

12

12

m

m

12

m

p

Amphiphilic water-soluble polymers and their hydrophobically driven self-associations have been studied extensively because of their potentials in industrial and biological applications (1-10). There is a class of hydrophobically modified water-soluble polymers known as associative thickeners (AT polymers) that are used in commercial applications for rheology control (11-22). AT polymers and their model polymers include copolymers of acrylamide or (meth)acrylic acid with associative comonomers containing an oligomeric ethylene oxide (EO) chain as a spacer between a polymerizable moiety and terminal hydrophobe (11-16,23). Associative comonomer units in such copolymers play a role as "stickers" in aqueous solutions. The content of associative comonomers in AT copolymers is usually very low (< 2 mol %) because AT polymers are

12

© 2003 American Chemical Society In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch002

13 designed, for a practical reason, so that interpolymer association can be achieved with a minimum number of hydrophobes incorporated in the polymer. Associative properties of hydrophobically modified polyelectrolytes strongly depend on polymer architecture. Polymer-bound hydrophobes can associate within the same polymer chain (intrapolymer association) or between different polymer chains (interpolymer association). In most cases, hydrophobically modified random copolymers undergo both intra- and interpolymer associations concurrently. The balance of intra- and interpolymer associations depends on polymer structures including the type and number of hydrophobes per polymer chain, distribution of hydrophobes along polymer chain, and distance of hydrophobe from the backbone (i.e., spacer) (20,24-36). In particular, spacer between hydrophobe and polymer backbone is an important parameter that controls the associative behavior of polymer-bound hydrophobes (30-36). This is due to the fact that in the event of self-association of polymer-bound hydrophobes, polymer chains impose steric constraints to the hydrophobes, restraining their motions. Interpolymer association is likely to occur more easily when the number of hydrophobes per polymer chain is smaller, whereas intrapolymer association becomes more favorable as the number of hydrophobes increases (31,32). In a polymer possessing a larger number of hydrophobes per chain, hydrophobes are located closer to others, and thus there is more chance for them to associate on the same polymer chain. As intrapolymer association proceeds, polymer-bound hydrophobes form clusters (i.e., hydrophobic microdomains). Thus, the intrapolymer association forces a polymer chain to collapse into a "closed" structure in which hydrophobes may no longer be available for association with other hydrophobes on different polymer chains. In a polymer with a small number of hydrophobes, on the other hand, hydrophobes are located far apart from others on the same polymer chain, a circumstance where intrapolymer hydrophobe association is unfavorable, and hence hydrophobes are available for interpolymer association This chapter deals with copolymers of sodium 2-(acrylamido)-2methylpropanesulfonate (AMPS) and a methacrylate substituted with HO(CH CH 0) Ci2H 5 (CnE*) (DBwMA) where m = 6 or 25 (Chart 1). A stmctural feature of these copolymers, comparing to AT polymers mentioned above, is that a much larger number of surfactant comonomer units are incorporated into a strong polyelectrolyte. Since the (CH CH 0) spacer is a flexible chain, the Ci group at an end of the spacer may be able to move freely depending on its length (m). In addition, the polyAMPS backbone and the EO spacer chain are highly hydrated in water and thus the terminal C i should be excluded from hydrophilic macromolecular surroundings. Therefore, Ci chains may easily associate, forming micelles. Micelles may be formed from associations of the surfactant moieties not only on the same polymer chain but also on different polymer chains, and therefore the micelles formed may be connected with polymer chains, forming a network structure. This chapter describes the side-chain micellization of polyAMPSbound CnE*, moieties and rheological properties of solutions of the polymer network. 2

2

ra

2

2

2

m

2

2

2

In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

14 Experimental


Synthesis and Characterization of Polymers. The surfactant monomers were synthesized by reacting CnEm (m = 6 or 25) with methacryloyl chloride in the presence of triemylainine in benzene (36,37). Copolymerization of a water-soluble monomer and a surfactant monomer can be performed in organic solution or in aqueous media in the presence or absence of nonpolymerizable surfactants (19,38-41). In aqueous media, "micelle" or "emulsion" polymerization may occur, by which copolymers with a blocky sequence distribution of the surfactant comonomer unit may be obtained (19,22,38-40,42). In contrast, homogeneous polymerization in organic solution may yield copolymers with a random distribution. The association behavior of the copolymer depends strongly on the sequence distribution of surfactant comonomer units (19,20,26,39,43,44). To obtain copolymers with a random distribution of AMPS and DEmMA (Chart 1), homogeneous solution polymerization was adopted using A^N-dimethylformamide (DMF) as a solvent in the presence of 2,2'-azobis(isobutyronitrile) at 60 °C (36). Copolymer compositions, determined by *H NMR spectroscopy, were virtually the same as monomer feed compositions (36). Apparent number- and weightaverage molecular weights of the copolymers, roughly estimated by gel permeation chromatography (GPC), are listed in Table 1. Numbers of surfactant units per polymer chain roughly calculated from copolymer compositions and number-average molecular weights are also given in Table 1.

Table I. Characteristics of a

EO length mol%

AMPS-DEJMMA

Copolymers

Hydrophobe per cmc polymer chain (z/L) m~6 0.012 10 4.8 6.5 2.4 20 17 6.9 0.022 2.7 10 m = 25 7 0.006 7.0 3.0 0.018 15 13 6.9 1.9 0.021 20 7.6 15 2.9 30 0.024 16 8.3 2.8 Comonomer content determined by H NMR in D2O. Determined by GPC using a 0.1 M U C I O 4 methanol solution as ement Standard PEO samples were used for molecular weight calibration. Determinedfromsteady-state fluorescence excitation spectra of pyrene probes. a

4b

M xl0

MJMn

w

l

b

c


15 Measurements. Steady-state fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Emission spectra of pyrene probes were measured with excitation at 337 nm at room temperature. Excitation spectra were monitored at 372 nm. Sample solutions were prepared by dissolving a predetermined amount of polymer in an aqueous pyrene solution of a known concentration, and the solutions were allowed to stand for 1 day for equilibration Fluorescence decay data were collected on a HORIBA NAES 550 system equipped with a flash lamp filled with H . Sample solutions were excited at 337 nm, and pyrenefluorescencewas monitored at 400 nm. Sample solutions were purged with Ar for about 30 min prior to measurement. Quasielastic light scattering (QELS) measurements were performed with an Otsuka Electronics Photal DLS-7000 light scattering spectrometer equipped with a 65-mW Ar ion laser (A = 488 nm). Data were collected using an ALV-5000 wide-band multi-T digital autocorrelator. All measurements were performed at 25 °C. Sample solutions were filtered prior to measurement using a 0.45-fim pore size membrane filter. Relaxation time distributions were obtained by an inverse Laplace transform analysis by conforming the REPES algorithm (36,37). Apparent hydiodynamic radii were calculated using the Einstein-Stokes equation. The rheological behavior of the copolymers was measured with a DynAlyser 100 stress-control rheometer (RJheoLogica) equipped with a cone and plate at 25 °C. The radius of the cone is 40 mm, and the angle between the cone and plate is 4.0°. Steady shear and oscillatory flow measurements were conducted to obtain the steady shear viscosity and dynamic viscoelastic properties of polymer solutions.


2

Results and Discussion

Apparent Critical Micelle Concentration (cmc) and Aggregation Number for Polymer-Bound Micelles. Since the micelle formation of pohyAMPS-bound CnE^ moieties in aqueous solutions is mainly due to interpolymer association, an apparent "cmc" is observed. Here, the apparent "cmc" is defined as a polymer concentration for the onset of interpolymer association. Apparent cmc for polyAMPS-bound CnE* moieties was estimated using fluorescence excitation spectra of pyrene probes (35,36). This method is based on the fact that the 0-0 absorption maximum for pyrene in water at 334 nm shifts to 337 nm when pyrene is solubilized in a micellar phase (45-48). The ratio of the intensity at 337 nm relative to that at 334 nm (hnlh^, estimated from excitation spectra, increases with polymer concentration (C ). As illustrated in Figure 1 for copolymers of m = 25 with varying C12E25 p


16

Chart 1. AMPS-DEwiMA Copolymers. CH - f C H — CH-)

(- C H — cA—

2

\

2

I hoo-x \ 0

=

c

J Ix

0=C

\

\

0-{cH CH 0 V ~ f C H ) — C H 'm '11

NH Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch002

3

2

2

2

v

H3C C CH3 —

—

CH S 0 " Na

3

x

2

+

3

AMPS-DE6MA: m = 6, X= 10 and 20 AMPS-DE25MA: m = 25, X= 10,15, 20, and 30

1.6

rmn|

1 1-1-1

i|

(mol %) ; fDE25

1.4

&=Sp»

=

•fro • •

10

S 1-2

J

10

1

20

5

-

-

0.8 -

-

tjd^S

0.6 .

....1

10

-3

r

. . ... i T

10

30 .i

. -2

10

_1

10°

•

• 1

10

Cp (g/L) Figure 1. Plots ofhv/hu as a function ofthe polymer concentration for the copolymers ofm = 25 with varying fm25*


17 surfactant comonomer contents (/DE2S) (Chart 1), hulhu starts to increases more significantly at a certain C with increasing C . A similar tendency was observed for copolymers of m = 6. From hnlhiA at a given C , along with minimum and maximum hwlhyi values observed at low and high C , respectively, one can calculate the ratio of pyrene concentrations in the micellar and aqueous phases ([Py] /[Py]w) at a given C . Figure 2 exhibits an example of (Py]m/[Py]w vs C plots for copolymers of m = 6 with varying Ci E surfactant comonomer contents (fo) (37). The plots comprise two linear lines with a smaller slope at lower C (inset of Figure 2) and a larger slope at higher C . Transition from the small to large slope is sufficiently abmpt to be able to define an apparent cmc. At low C , surfactant moieties associate within the same polymer chain independent of C , forming intrapolymer hydrophobic domains. Upon increase in C to a certain level, however, interpolymer association commences to occur. From a break observed in the [Py]m/[Py]w vs C plot, apparent cmc was estimated, as listed in Table 1. The slope of the linear line corresponds to partition coefficient (Kv) for pyrene solubilization; the steeper the slope, the stronger ability to solubilize pyrene in the micellar phase (35-37). Thus, micelles formed at C > cmc are very differentfromintrapolymer hydrophobic domains formed at C < cmc in ability to solubilize pyrene, probably arising from a difference in the size of hydrophobic cores (i.e., aggregation number). In Figure 3, cmc and K values are plotted against fme and fmis for the copolymers of m - 6 and 25, respectively (37). Values of cmc are represented as the molar concentration of CnEm residues in the copolymers. Values of cmc for free C i Ee and C E 5 surfactant molecules in 0.1 M NaCl aqueous solutions were estimated to be 6.0 x 10" and 3.0 x 10" M, respectively by the same method based on the excitation spectra of solubilized pyrene probes (36,37). Values of cmc for the copolymers are smaller than cmc for the corresponding free CnE surfactants by more than 1 order of magnitude. The cmc values for the copolymers show a tendency to increase with increasing content of surfactant units in the copolymers (Figure 3). A plausible cause is that as the content of the surfactant units in the copolymer increases, the polymer-bound surfactants are more prone to associate on the same polymer chain and thus the number of surfactants available for interpolymer association decreases. This idea is supported by the fact that the slope of the [Py]m/[Py]w vs C plot at lower C increases with increasing surfactant content in the copolymer (inset of Figure 2). K values are fairly constant on the order of 10 for the two copolymers (Figure 3). These values are a slightly smaller than K values for the corresponding free CnE™ micelles. Aggregation numbers (N^) of dodecyl groups in one core of polymerbound CnEm micelles were determined by a fluorescence technique based on the excimer formation of pyrene probes solubilized in the polymer-bound micelles (49-52). When the concentration of solubilized pyrene is sufficiently low, pyrene emits only monomeric fluorescence, exhibiting a single-exponential decay. However, if pyrene concentration is increased to a certain level, pyrene emits excimer fluorescence, showing a fast decay component because the probability of finding two or more pyrene molecules in the same micelle core increases. In this situation, fluorescence decay p

p

p

p

p

m

p

2

6

p

p

p

p


p

p

p

p

y

2

12

2

5

4

m

p

p

v

y


18

4.0

0.12

0 'DEB

(mol%) =

_ 0.08

3.0

/

20

/

• 0.04

(mol %)

2.0

.

20,,

«

0

0.02

0.04

Q.

10


1.0

O-I-X-LI

0.1

i

• i

0.2

I • .

. . i

.

0.4

0.3

0.5

C (g/L) p

Figure 2. Plots of[Py]J[Py]y, as a function of the polymer concentration for the copolymers ofm = 6 with foE6 = 10 and 20 mol %.

"•

C

P

1

-i

-

r discrete • C E25

discrete

3

C

12

E

12 25

LU CM

/77 = 6 ^

10"

discrete j 12 6

m = 25

(

C

"discrete

d

10°

E

^12^6

o

10r5

QL

10"

E o c £ CO

.10"

0

10

f

DE6

or / D

20

30

5

(mol %)

E 2

40

Figure 3. Plots of apparent cmc and partition coefficient (K,) for the copolymers ofm = 6 and 25 as a function ofmacromonomer content in the copolymers. The apparent cmc is represented in terms of the molar concentration of the CirfZm unit.


19 profiles may follow a kinetic model developed by Infelta (53) and Tachiya (54)', ln[7(0//(0)] =A [cxp(-A 1) -l]-A t 3

4

(1)

2

A = ko + n kQk l(kq+k~) 2

(2a)

Q

2

A3 = n k /(kQ + k-f

(2b)

A = kQ + k

(2c)


Q

Q

4

where, 1(f) and 1(0) are the fluorescence intensities at time t and 0 following pulse excitation, respectively, ICQ is the pseudo-first-order rate constant for fluorescence quenching, fa is the fluorescence decay rate constant (i.e., to") for pyrene inside the micelle without excimer formation, n is the average number of quenchers in a micelle, and AT is the first-order rate constant for exit of pyrene molecules from a micelle. w can be expressed as [Q] /[M], where [Q] is the molar concentration of quencher inside micelle and [M] is the molar concentration of the micelle. Because the quenching of pyrene monomer fluorescence is due to the excimer formation of pyrene, «Q is the average number of pyrene molecules solubilized in a micelle. Assuming complete solubilization of pyrene, one can calculate the molar concentration of micelles from which one can calculate using n determined from best fit of fluorescence decay data to eq 1 (35-37). Values of Mgg thus estimated are plotted in Figure 4 as a function of the molar concentration of the surfactant unit in the copolymers (36,37). In Figure 4, N values for micelles formed from free C nEm molecules are also plotted against the surfactant concentration. The values of for the copolymer of m = 6 range from ca. 170 to ca. 270, increasing with the concentration of the surfactant unit in the copolymer (i.e., polymer concentration). A similar trend was observed for free G 2 E 5 surfactant molecules. On the other hand, the values of for the copolymer of m = 25 are nearly constant at ca. 57. A similar trend was observed for free C12E25 surfactant molecules. These A ^ values for the two copolymers are fairly close to those of the corresponding discrete micelles of free CnEm surfactant molecules, and the A ^ values for the copolymer of m = 25 are much smaller than those for the copolymer of m = 6. values for polymer-bound C i E micelles are either smaller or larger than those for free C12E6 micelles depending on the polymer concentration, whereas Afagg values for polymer-bound G2E25 are slightly larger than those of free G2E25 micelles over the range of the surfactant concentrations studied. These aggregation numbers for the polymer-bound micelles are much larger than the numbers of surfactant units per polymer chain (Table 1). Therefore, it is evident that micelles are formed from associations of the surfactant moieties not only on the same polymer chain but also on different polymer chains. For example, in the case of the copolymer of m = 6 with 1

Q

Q

m

m

Q

m

2

6



20

350 r 300 250 - m = S 200 O)

150discrete C E 5 -

100 -- m = 25

12

/

50 0^

2

10^

10"

-2

10

[C E unit] (M) 12

m

Figure 4. Plots ofN as a function of the molar concentration of the C i2E unit for the copolymers ofm = 6 and 25 with x = 20 mol % in 0.1 MNaCl aqueous solutions. N for discrete Ci2Em surfactants are also presented. agg

m

agg


/DE6 = 20 mol %, 1 polymer chain possesses 17 Ci E units (Table 1) while Wagg is 243 at C = 6.25 g/L. Thus, 1 micelle is formed by at least 14 polymer chains assuming an extreme case where all the CnEe units on the same polymer chain belong to the same micelle. In the case of the copolymer of m = 25 w i t h e s = 20 mol %, 1 micelle is formed by at least 4 polymer chains assuming the same extreme case. As is evident from QELS and rheology data, polymer chains are crosslinked by micelles, as will be discussed later. The occurrence of crosslinking indicates that surfactant units on the same polymer chain occupy different micelles. Therefore, each micelle is formed from a larger number of polymer chains than those calculated above. 2

6


p

Hydrodynamic Size of Polymers Crosslinked by Micelles. In the case of the copolymer of m = 25 (fes = 20 mol %), distributions of QELS relaxation times in 0.1 M NaCl aqueous solutions are bimodal at C ^ 18.8 g/L but they become unimodal at higher C (Figure 5). A similar tendency was observed for the copolymer of m = 6 (T E6 = 20 mol %) but the presence of a fast mode persists to higher C than for the copolymer of m = 25 (Figure 5). The slow relaxation mode observed in the semidilute regime shown in Figure 5 is attributed to polymers crosslinked by C^E^ micelles, and the fast-mode component may be attributed to a unimer (single polymer state) or an aggregate of a small number of polymer chains ("oligomeric" micelle). There is a clear trend that peak top for the slow mode shifts toward longer relaxation times with increasing C . Apparent hydrodynamic radius (Rh) for the crosslinked polymer chains estimated from QELS data increases gradually with increasing C in a low C region but Rh increases markedly with C in a higher C region. Although polymer-bound C i ^ micelle cores conserve their size over a significant range of C (Figure 4), hydrodynamic size of the crosslinked polymer chains increases significantly with increasing C . This means that the degree of crosslinking increases with C whereas the size of micelles formed from polymer-bound C nE™ moieties (i.e., JVagg) remains unchanged. p

p

D

p

p

p

p

p

p

p

p

p

Steady Shear Solution Viscosities of Polymers Crosslinked by Micelles. Zero-shear viscosities of the copolymers of m = 6 (/DE6 = 20 mol %) and 25 (/" E25 = 20 mol %) in 0.10 M NaCl aqueous solutions are plotted in Figure 6 as a function of C (37,55). The viscosity increases gradually with C in a low C region. In this low C region, the degree of crosslinking of polymer chains by micelles is relatively small. However, viscosity starts to increase sharply as C is increased to ca 10 g/L for m = 25 and to ca, 30 g/L for m = 6. In this C region, the degree of crosslinking increases greatly with C , yielding a macroscopic network. Viscosities for the copolymer of m = 25 are roughly 2 orders of magnitude higher than those of the copolymer of m = 6 at a given C in this region; the size of the network of the copolymer of m = 25 is much larger than that of the copolymer of m = 6. D

p

p

p

p

p

p

p

p


22


f

15

DE6

= 20 mol %

18.8

g/L

25.0

g/L

/\

,

ml

37.5

g/L

50.0

g/L

• .1

1

f

d

DE25

i mill

= 20 mol %

i 11 m i l l

* •

1

10'

2

1

10'

10°

1

10

10

2

2

1

lO^O" 10"

Relaxation time,

10°

' • •

1

1

10

10

2

10

r(ms)

Figure 5. Relaxation time distributions in QELS at 6 = 90°for the copolymers ofm = 6 and 25 with x = 20 mol % in 0.1 MNaCl aqueous solutions at varying polymer concentrations.



23

Figure 6. Plots ofzero shear viscosity at 25 °Cfor the copolymers ofm = 6 and 25 with x = 20 mol % in 0.1 MNaCl aqueous solutions as a function of the polymer concentration.


24 Zero-shear viscosity depends on the concentration of added salt, and salt-dependent viscosity profiles for the copolymers with m = 6 and 25 are greatly different (Figure 7). In the case of m = 6, viscosity increases rather gradually with increasing salt concentration ([NaCl]) at C s 125 g/L, but it shows a more significant increase at C = 150 g/L with increasing [NaCl] at [NaCl] > 0.2 M. In the case of m = 25, in contrast, viscosity shows a maximum at a certain salt concentration depending on C (36b). The viscosity first increases with [NaCl], reaching a maximum and then decreases with further increasing [NaCl]. The viscosity maximum occurs at [NaCl] = 0.10 - 0.15 M at C ^ 50.0 g/L, and the maximum shifts toward higher salt concentrations at higher polymer concentrations (C s> 75.0 g/L) (Figure 7b). This viscosity increase with [NaCl] is more significant at higher C , followed by a sharp drop in viscosity after passing through a maximum. These observations may be explained as follows (36b). Polymer chains are crosslinked when surfactant groups on the same polymer chain occupy concurrently two or more micelles formed from surfactant groups on different polymer chains. This process is an interpolymer association that occurs competing with interpolymer electrostatic repulsion and hence occurs more favorably at higher salt concentrations because of increased electrostatic shielding. Association of surfactant groups on the same polymer chain (i.e., intrapolymer associations) also becomes more favorable at higher salt concentrations because of an increased short range electrostatic shielding within the same polymer chain. Thus, these two kinds of salt effects, competing in the process of micelle formation, may give rise to a maximum degree of crosslinking at a certain salt concentration. In the case of m = 6, the second salt effect appears to be less pronounced than in the case of m = 25 because surfactant groups in the copolymer of m = 6 are more prone to associate within the same polymer chain than in the copolymer of m = 25 as indicated by QELS and viscosity data. Therefore, only the first salt effect is visible in the viscosity data (Figure 7a). In contrast, for m = 25, intrapolymer association becomes more pronounced at higher salt concentrations and therefore the second salt effect predominates over thefirstsalt effect at higher salt concentrations showing a maximum in viscosity vs [NaCl] plots. p

p

p

p

p


p

Shear-Rate-Dependent Solution Viscosities of Polymers Crosslinked by Micelles. Copolymers with m = 6 and 25 exhibit shear-rate-dependent viscosities in the presence of salt Steady-shear viscosities of the copolymers of m = 6 (fim = 20 mol %) and 25 (fam = 20 mol %) in aqueous solutions containing different concentrations of NaCl are plotted in Figure 8 as a function of shear rate (36b). Figure 8a is an example for the copolymer of m = 6 at C = 125 g/L. The polymer solutions exhibit Newtonian behavior at shear rates < ca. 100 s", and viscosity in the Newtonian region increases with increasing [NaCl] due to an increased degree of crosslinking with increasing [NaCl], as discussed above. At shear rates > ca 100 s", however, polymer solutions undergo shear thinning. The extent of the shear tMnning is more pronounced for solutions with higher Newtonian viscosities at higher salt p

1

1


25


10

I

20

In 15 CO

10 5 0

1

1

'

1

l

\

1

1

'

1

I

1

1

'

1

l

1

1

1

1

l

1

'

1

'

(g/L) =

•(b)

\ 75.0

/

J '

.50.0

'

2 5 . 0 \ ^

0.1

0.2

•

100

\ :

\

0.3

0.4

0.5

[NaCl] (M) Figure 7. Zero shear viscosities at 25 °Cfor the copolymers ofm = 6 and 25 with x = 20 mol % at varying polymer concentrations plotted against salt concentration.


26

1.4

CO

I

I 1111

I

1.0 CO

8

• •

I

I

I I I III)

I

I

I I

[NaCl] = 0.40 M

1.2

CD


I I I

0.20 M j-pQCPDCCHJlXJUi-Lja

0.8

CO

•> 0.6

0.10 M n - oOOOooOOOOOOOOCXDOOOOOOOOOOOCO ) OOCOOOOOOCC ca. 0.1 M, and the extent of the shear thickening increases as [NaCl] is increased. When the shear rate is further increased beyond 200 s , shear thinning occurs. Given that shear-thickening is derived by a shear-induced increase in the density of mechanically active chains, a plausible cause for the observed shear thickening is a shear-induced increase in the degree of crosslinking (50,56). When shear stress is applied to polymer chains crosslinked by micelles at a shear rate beyond the Newtonian region, polymer chains may be stretched out causing some surfactant groups to be pulled out of the micelle and become available for associations with other surfactant groups on different polymer chains, leading to an increase in crosslinking sites and hence a viscosity increase. A further increase in the shear rate may cause the crosslinking site to fragment, resulting in a decrease in the size of network structures and hence a viscosity decrease. The viscosity restored instantly when applied shear stress was removed, indicating that the shear-induced disraption and re-formation of crosslinking are reversible, as conceptually illustrated in Figure 9. This is a characteristic feature for reversible transient networks (36b). When a polymer network is sheared at a high shear rate, crosslinks are disrupted at a faster rate than the rate of their re-formation, leading to a decrease in steady shear viscosity. l

p

p

p


1

1

Viscoelastic Behavior of Polymer Networks. Figure 10 shows plots of storage modulus (G') and loss modulus (G") as a function of angular frequency (co) for the copolymers of m = 6 (f = 20 mol %) and 25 (fans = 20 mol %) at varying polymer concentrations at [NaCl] = 0.1 M (36b,37). In the case of the copolymer of m = 6, solutions exhibit viscoelastic properties at C < 125 g/L, and the G'-co and G"-co plots can befittedreasonably well to a Maxwell model with G" and G' being proportional to co andft?,respectively. As C is increased, both G' and G" increase and they become close to each other, and solutions exhibit significantly elastic properties. In the case of the copolymer of m = 25, solutions behave as a viscoelastic liquid, G* and G" data following a Maxwell model at C s 25.0 g/L. When C is increased to 50.0 g/L, both G' and G" increase by about 1 order of magnitude, and the G'-co and G"-co plots deviate from the Maxwell model. As C is further increased to 150 g/L, G' and G" become the same DE6

p

p

p

p

p



28

Figure 9. Conceptual illustration ofreversible disruption and re-formation of crosslinks under shear conditions.



29

co (rad/s)

Figure 10. Plots ofstorage (G') and loss (G") moduli as a function of angularfrequency(co) for the copolymers ofm = 6 and 25 with x = 20 mol % at varying polymer concentrations in 0.1 MNaCl. Closed and open symbols represent G' and G", respectively. Shear stress applied is 1.0 Pa at25°C


30 order of magnitude, and they are nearly parallel at low and intermediate frequencies, solutions exhibiting significant elastic properties. According to a simple theory of rubber elasticity extended to transient networks or reversible physical crosslinks (57), the magnitude of G' at a moderate co value is related to the density of mechanically reversible physical crosslinks. On the other hand, an increase in G" indicates an increase in effective volume occupied by polymer networks in solutioa At C = 150 g/L for the copolymer of m = 25, the slope of the G'-co plot is rather flat over the co range in Figure 10b, exhibiting gel-like behavior (58). The dominant elastic properties in the concentrated regime arise from a large number of crosslink junctions by C12E25 micelles. According to the Maxwell model, G' and G" are described by a plateau modulus (Go) and a terminal relaxation time (A) (58) as Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch002

p

2

2

2

2

G' = G eo A /(l + o>A)

(3)

0

2

G" = G ft)A/(l + co ??)

(4)

0

G and A for the copolymer of m = 6 at C = 100 g/L at varying salt concentrations were estimated by fitting G' and G" data to eqs 3 and 4. At a higher polymer concentration (C = 150 g/L), at which G' and G" data do not follow the Maxwell model, G and A were roughly estimated from the intersection of extrapolated lines for G' and G" plots. Figure 11 shows plots of Go and A thus estimated as a function of salt concentration. Values of Go at C = 150 g/L are more than 1 order of magnitude larger than those at C = 100 g/L. G increases with increasing salt concentration, and this tendency is more significant at the higher polymer concentration. In contrast, values of A at C = 100 and 150 g/L are close to each other on the order of 40 ms and slightly decrease with increasing salt concentration. Values of G and A for the copolymer of m = 25 at C = 25.0 and 75.0 g/L at varying salt concentrations were also estimated from G' and G" data and plotted as a function of the salt concentration in Figure 12 (36b). At C = 25.0 g/L, Go increases with increasing salt concentration passing through a rnaximum at a salt concentration near 0.13 M and then decreases (Figure 12a). A similar tendency is observed for C = 75.0 g/L but the maximum shifts toward higher salt concentrations ([NaCl] * 0.2 M) and G drops sharply at [NaCl] > 0.2 M (Figure 12b). On the other hand, values of A at C = 25.0 and 75.0 g/L are on the order of 10 ms decreasing only slightly with salt concentration. Zero shear viscosity can be related to Go and A as ?]o - GoA. The profiles of rj vs [NaCl] plots in Figure 7 are quite similar to those of G vs [NaCl] plots in Figure 12. Furthermore, viscosity values calculated from t} = G A at varying salt concentrations are in fair agreement with experimental values. Therefore, it is obvious that viscosity is essentially governed by G , and A has little or no effect on viscosity. The simple theory for transient networks (57) suggests that the magnitude of G is proportional to the number density of mechanically active chains in the network. Crosslinks formed by micelles act as transient junctions for a network structure, the junctions being in equihbrium between disruption and re-formation (Figure 0

p

p

0

p

p

0

p

0

p

p

p

0

p

0

0

0

0

0

0



31

Figure 11. Dependence ofplateau modulus (Go) and terminal relaxation time (A) on salt concentration for the copolymer ofm = 6 with f 6 = 20 mol % at (a) C = 100 g/L and (b) C = 150 g/L. DE

p

p


32

50 C = 75.0 g/L p

40


30 to

E, 20

N

10 50 C = 25.0 g/L p

40 30 w 20 10 0.1

0.2

0.3

0.4

0.5

[NaCl] (M) Figure 12. Dependence ofplateau modulus (GQ) and terminal relaxation time (}) on salt concentration for the copolymer ofm = 25 with fm2$ - 20 mol % at (a) C = 25.0 g/L and (b) C„= 75.0 g/L. p


9). The lifetime of the crosslinks may depend on the residence time for a polymer-bound surfactant unit in a micelle. The terminal relaxation times may be related to the residence lifetime of the surfactant unit in the micelle under shear conditions. Consequently, in the case of the copolymer of m = 6, the number of crosslinking junctions increases with increasing salt concentration, leading to an increase in zero shear viscosity. In the case of the copolymer of m = 25, however, the number of junctions increases with increasing salt concentration in a region of low salt concentrations (e.g., [NaCl] < ca 0.13 M for C = 25.0 g/L), but with a further increase in the salt concentration beyond a certain level, the number of junctions decreases with salt concentration. In both the cases, the lifetime of the junction remains practically the same independent of the polymer concentration over the salt concentrations examined. Micelles formed from O2E25 may be more dynamic in nature than micelles formed from CiJEe because the hydrophilic EO chain in C12E25 is more than 4 times longer than that in Ci E . The longer EO chain in C 1 2 E 2 5 is responsible for the higher cmc (Figure 3) and smaller for G2E25 micelles (Figure 4) than those for CnEe micelles. The relaxation times for the copolymer of m = 6 are about 4 times longer than those for the copolymer of m = 25. This is probably due to the nature of CnEe micelles as compared to C12E25 micelles.


p

2

6

Concluding Remarks Because of their relevance in molecular organization in biological systems and also to various practical applications, functionalization of polyelectrolytes with various hydrophobes has been and will continue to be an area of great scientific and technological interest. A key to designing a hydrophobically modified polyelectrolyte for a given application is a fundamental understanding of the structure-property relationship. This chapter has focused on micelle formation of C12EL surfactant moieties covalently linked to poly AMPS and rheological behavior in water of polymer networks crosslinked by micelles. The degree of crosslinking and the tenninal relaxation time depend strongly on the length of EO chain in the CnBm surfactant moieties. The transient nature of the crosslinking junctions originating from the dynamic nature of the micelles dictates dynamic viscosity and viscoelastic behavior of solutions of the polymer networks.

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